1. Field of the Invention
The present invention relates to dielectric filters and dielectric duplexers, and more particularly, to dielectric filters and dielectric duplexers in which a plurality of dielectric resonators are provided in a single dielectric block.
2. Related Art
A known dielectric filter in which a plurality of dielectric resonators are provided in a single dielectric block is shown in FIG. 18. In this dielectric filter, two resonator holes 32a and 32b pass through opposing surfaces 31a and 31b of a dielectric block 31. The resonator holes 32a and 32b have large-diameter hole sections 42a and 42b, and small-diameter hole sections 43a and 43b connecting to the large-diameter hole sections 42a and 42b. The axes of the small-diameter hole sections 43a and 43b are eccentrically shifted from those of the large-diameter hole sections 42a and 42b. In other words, as shown in FIG. 19, the axes of the small-diameter hole sections 43a and 43b are shifted a distance P from those of the large-diameter hole sections 42a and 42b wherein P is within a range defined by 0&lt;P.ltoreq.R-r, where R indicates the radius of the large-diameter hole section 42a and 42b, r indicates the radius of the small-diameter hole section 43a and 43b, and P indicates the shift distance between the respective axes of the large-diameter hole sections 42a and 42b and those of the small-diameter hole sections 43a and 43b (see FIG. 19).
An outer conductor 34 is formed on almost all the outer surface of the dielectric block 31. One pair of input and output electrodes 35 is formed on the outer surface of the dielectric block 31. The pair of electrodes 35 are not electrically connected to the outer conductor 34 because of a gap maintained between them. Inner conductors 33 are formed on almost all the surface inside the resonator holes 32a and 32b. Gaps 38 are provided between the inner conductors 33 and the portions of the outer conductor 34 extending into the openings of the large-diameter hole sections 42a and 42b.
In the known dielectric filter having the structure described above, as shown in FIG. 19, when the distance d1 between the axes of the small-diameter hole sections 43a and 43b is set longer than the distance d2 between the axes of the large-diameter hole sections 42a and 42b, the electromagnetic coupling between the resonator holes 32a and 32b becomes capacitive coupling. Conversely, when the distance d1 between the axes of the small-diameter hole sections 43a and 43b is set shorter than the distance d2 between the axes of the large-diameter hole sections 42a and 42b, the electromagnetic coupling between the resonator holes 32a and 32b becomes inductive coupling. The level of the electromagnetic coupling between the resonator holes 32a and 32b is set to the desired strength by changing the distance d1 between the axes of the small-diameter hole sections 43a and 43b.
However, since the axes of the small-diameter hole sections 43a and 43b are shifted eccentrically to those of the large-diameter hole sections 42a and 42b only in a range of 0&lt;P.ltoreq.R-r in the conventional dielectric filter, the range over which the distance d1 between the axes of the small-diameter hole sections 43a and 43b can be varied is narrow. Therefore, the strength of the level of the electromagnetic coupling between the adjacent resonator holes 32a and 32b cannot be varied over a wide range. Consequently, when a stronger electromagnetic coupling is required between the adjacent resonator holes 32a and 32b, the external shape and dimensions of the dielectric block 31 need to be changed.